Transparency adds to file size as per every pixel it has to store data for RGBA instead of just RGA so thats an extra bit every pixel but we are not talking much here.
Jpegs are generally smaller and if you have access to photoshop or the like (gimp might do it) you can compress them in that and reduce the quality in trade for a reduced file size. No need to really go over 75% and you can keep reducing it way beyond that, it's only things like gradients that tend suffer. Try also reducing the colour count to see what you can get away with.
Reducing the size of the sprite (outside of constuct*) will also reduce the filesize. Less pixels means less data needed to be stored however when you scale the images back up you will see a noticable difference in quality.
Other things that you could try is to repeat the use of sprites. Using 7 different tree images? Break it down in a few parts and just make up lots of tree's using a combination of the pieces you have. Rotate/flip/scale to create variation. Use actions to animate it and remove frames from sprites.
*edited in light of mod comments see below